1. Field
This disclosure relates generally to the field of hydrated glass materials and specifically to hydrated glasses having properties useful for optical applications.
2. Prior Art
The hydration of various glass compositions is well known. See, for example, U.S. Pat. No. 3,498,802 and U.S. Pat. No. 3,489,803, both of which disclose how low temperature thermoplastic-type behavior can be imparted to glass by incorporating varying amounts of water within the glass structure. See also U.S. Pat. No. 3,912,481 which discloses a hydration-dehydration process wherein the importance of carefully controlling the final water content is disclosed.
The low temperature thermoplastic-type behavior associated with certain hydrated glasses has suggested various practical applications where the desirable properties of glass and low temperature thermoplasticity can be advantageously combined. Recently, in patent applications Ser. No. 640,733 and Ser. No. 640,735, both cited above as related applications, it has been disclosed that under defined conditions, it is possible to mold certain hydrated glass compositions in such a manner that an optical quality surface on a die is transferred to the molded article. This permits the preparation of an optical quality surface on a glass body without the need for grinding and polishing steps, thus suggesting numerous economies in optical components manufacture.
In Ser. No. 640,733, as well as this application, the expression "optical quality surface", or its equivalent, refers to a surface having a "Roughness Height" the arithmetic average (AA) of which is less than 3.0 microinches (3.0.mu. inches) over a distance of at least 0.1 inch. A very preferred optical quality surface has a Roughness Height, the AA of which is less than 1.6.mu. inches over the same distance. The expression "Roughness Height", for purposes of defining a standard (of surface smoothness), is described as the arithmetic average (AA) deviation (of a surface instrument trace) expressed in microinches (.mu. inches) measured normal to the centerline. Arithmetic Average (AA) is also known in British Standards as Center Line Average (CLA). This and related terminology is explained more fully in the publication, Surface Texture, ASAB 46.1 -- 1962, published by the American Society of Mechanical Engineers, New York, N.Y.
Various instruments which can measure surface smoothness in the above terms are known (e.g. Proficorder.RTM., Surfanalyzer.RTM.). Detailed descriptions concerning the use of such instruments can be found in manuals used with the instruments and other publications. According to one such manual (Gould Surfanalyzer.RTM., Model 1200), the results of surface measurement, expressed in AA, can be converted approximately to the root means square (rms) average by multiplying the AA by 1.11. Hence, surface smoothness can be expressed in AA units or rms units over a given surface. An example of rms measurements is described in an article entitled, "Polishing of Supersmooth Metal Mirrors", Applied Optics, Vol. 14, No. 8, pp. 1808-1812, August, 1975. See also an article entitled, "Surface Characterization: A Total Approach", Research/Development, November, 1975.
Although the disclosure of Ser. No. 640,733 is concerned primarily with the preparation of an optical quality surface in a glass article via molding techniques, it was noted that the articles molded had a relatively high light transmittance, compared with known sintered inorganics. For example, in one molded disc made according to the above disclosure, a value of 89% total transmittance (as defined in ASTM D 1003) was obtained. That disc, however, demonstrated a 2 to 3% backward scattering from the bulk (interior), and 5 to 20% of the light beam intensity was deflected 0.1.degree. to 10.degree. due to bulk inhomogeneity. Although such bulk inhomogeneity can be tolerated in lower grade optical articles or articles such as reflecting mirrors where only the surface smoothness must have an optical quality, it can be appreciated that a high quality optical article used for transmittance of light should permit near theoretical transmittance of light -- e.g. the article should be capable of transmitting that amount of light which approaches or is equal to the maximum amount theoretically possible for the glass. Such high degree of transmissivity (about 92%) requires a substantially uniform refractive index throughout the bulk of the glass and optical quality surfaces on the glass. A constant refractive index requires an extremely high degree of bulk homogeneity in the glass. In the case of a hydrated glass, this means that there must be no water concentration gradients in the glass bulk and total water content must be homogeneously distributed therein.
The present disclosure is concerned with an improvement over the disclosures cited in Ser. No. 640,733 in that it has been found that by carefully controlling a dehydration step and by molding a preformed shape, it is possible to achieve not only an optical quality surface and a given figure, but also a bulk (interior) capable of transmitting an amount of light substantially equal to the amount theoretically possible. Also, the surface figure of the die can be transferred to the glass. Details of our methods are described herein.